Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.
In one known type of variable geometry turbine an axially moveable wall member, generally referred to as a “nozzle ring” defines one wall of the inlet passageway. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as gas flow through the turbine decreases, the inlet passageway width may be decreased to maintain gas velocity and optimise turbine output. The nozzle ring is provided with vanes which extend into the inlet and through slots provided in a “shroud” defining the facing wall of the inlet passageway to accommodate movement of the nozzle ring. The vanes are at a fixed angle relative to the radius of the nozzle ring. A variable geometry turbocharger including such a variable geometry turbine is for instance disclosed in U.S. Pat. No. 5,868,552.
In different arrangements the nozzle ring is fixed and the shroud is axially moveable so as to control the axial width of the inlet passageway.
In current variable geometry turbochargers the moveable wall member (i.e. the nozzle ring or the shroud) is mounted on the end of push rods that are attached, at their other end, to a rotating yoke pivoting in an arc about a cross shaft. The yoke is arranged such that its rotation moves the push rods in the axial direction, so as to move the moveable wall member in the axial direction.
The yoke and push rods are mounted internally to the bearing housing. In this case, seals are required where the push rods pass through the bearing housing wall into the turbine housing. These seals are relatively expensive and require water cooling, which increases the complexity and cost of the variable geometry to the charger.
In an alternative arrangement, the push rods and the yoke are mounted in a cavity in the turbine housing. This arrangement does not require the seals of the bearing housing mounted arrangement. However, this arrangement is large and bulky in nature to allow for the mounting of the moveable wall member, push rods, and yoke. This results in a large overall turbine housing, which also increases its thermal inertia. Furthermore, due to the large cavities required to house all of the moving components, an increased pressure differential is experienced during filling and scavenging of the exhaust gas in the housing.